Brittonia 39: 220. 1987
Stems with cortical cells small, walls somewhat thicker than those of central cells. Leaves loosely erect-spreading, somewhat distant, incurved when dry, lightly to strongly falcate-secund near stem apices, concave in proximal 2/3, not undulate or plicate, 0.9–1.5 mm; marginal teeth single or double, often tipped with 1 papilla; apex gradually or sometimes abruptly narrowing, acumen flat, sharply pointed; costa slender, cristate distally; distal laminal cells linear-rhomboidal, 7.5–10:1, pellucid, walls thin. Seta 0.6–1.2 cm. Capsule black, 0.7–1.5 mm. Calyptra covering distal portion of urn. Spores 8–11 µm, smooth.
Phenology: Capsules mature spring (early Mar).
Habitat: Rock, wet places on soil, tree bases, wet, rotted wood, near spring seepage and stream banks, deep shade in wet forests, concrete culvert around springs, cut-over woods of Fagus, Magnolia grandiflora, Pinus
Elevation: moderate elevations
La., Miss., Mexico (Chiapas, Oaxaca, Puebla, Veracruz), South America, Pacific Islands (Cocos Island).
Trachyxiphium hypnaceum has been reported as abundantly fruiting (W. D. Reese 1974). This species is rare in the flora area, occurring in one parish (Washington) in Louisiana and two counties (Franklin and Washington) in Mississippi (C. Delgadillo M., pers. comm.). Trachyxiphium hypnaceum is easily recognized by its erect-spreading, distally secund leaves, which are lanceolate-acuminate with smooth and narrow cells, and the dorsally strongly serrulate costae. Callicostella pallida is similar in its somewhat thinner but similarly robust double costa and broad leaves lacking a margin of differentiated cells, but C. pallida leaves are lingulate with an abrupt apiculus (not long-acuminate) and have isodiametric laminal cells with a central papilla and firm walls (not linear-rhomboidal and thin-walled). W. D. Reese and B. E. Lemmon (1970) suggested that the isolated occurrence of species of Pilotrichaceae indicates chance introduction. Reese noted that collections of Trachyxiphium were associated with human activity, namely a fabricated lake in Mississippi and concrete culvert in Louisiana.